GMC 4000 Series Service Manual page 227

Sec. 5B
Page 220
AIR BRAKES
Figure 14 — Trailer Hose Coupling
foot brake system or by the trailer brake hand
control valve. Flexible hoses are used to make the
connection between the truck and the trailer. Each
hose is equipped with couplings for convenience in
connecting and disconnecting the truck and trailer
brake system s (fig. 14).
The emergency line is at the right side and is
tagged "Emergency." The service line is at the
left side and is tagged "Service." When coupling
tractor and trailer make sure that air lines are
properly connected. Couplings on trailer are also
tagged "Service" and "Emergency." See figure 15
for illustration of trailer connections mounted on
the back of a conventional cab model.
1. Couplings
Figure 14 shows a sectional view of a single
hose coupling, and a view of the two couplings
connected. The design of the coupling is such that
when the two are connected together, pressure is
placed on two rubber gaskets, making an airtight
seal. The rubber gaskets should be replaced every
six months, or more often under severe operating
conditions.
2. Dummy Couplings
Dummy couplings are mounted on rear of cab
and serve as hangers for the trailer hose when
they are disconnected from the trailer. Both hoses
should always be connected to the dummy coup­
lings when not connected to the trailer.
3. Cut-out Cocks (When Used)
Cut-out cocks are used in the trailer brake
lines on vehicles not equipped with trailer break­
away valve. Purpose of cut-out cocks is to provide
a means of closing off these lines when they are
not being used. The cut-out cock is open when the
handle is at a 90-degree angle to the body, and is
G M C S E R V I C E M A N U A L
1 Trailer Hose
2 Dummy Couplings
3 Brake and Electrical
closed when the handle is parallel with the body.
Stops are provided so the handle cannot be turned
beyond its normal open and closed positions.
for leakage. Connect cut-out cock to source of air
pressure, build up 90 pounds pressure against it,
and cover opening with soap suds. If leakage ex­
ceeds a 1-inch bubble in 3 seconds, cock should be
repaired or replaced. In some Instances, leakage
may be caused by dirt on the key. This condition
can be remedied by cleaning the key and seat with
gasoline, then grinding the key to its seat with
grade 400 grinding compound. Thoroughly wash off
all grinding compound and coat key lightly with a
good grade of cup grease before assembling. If the
key or body is badly scored, the complete cut-out
cock should be replaced.
provides the driver with a means of checking sy s­
tem air pressure. The vehicle should never be put
in motion until pressure reaches 65 pounds. If
pressure reading drops to less than 65 pounds
while vehicle is in motion, vehicle should be stop­
ped and the cause of air loss corrected. Dash
4 Hose Support Springs
5 Electrical Connections
Couplings
6 Tractor Protection
Lines
Figure 15— Trailer Brake Connections
Cut-out cocks should be tested periodically
AIR PRESSURE GAUGE
The air pressure gauge in the instrument panel
Valve